In the metal packaging industry today, can lids or deep-drawn cans made of steel or aluminum, in round or non-round shape, are often produced on high-performance systems. For reasons of productivity, portal presses are used, enabling the use of multiple tools. Depending on the type of press and the available pressing force, tools in a “zigzag” or linear arrangement are used. With each press stroke, several components are thus produced at the same time. The basic material—whether made of aluminum or steel—is processed in the press in the form of metal sheets. By means of one or more gripper systems or rollers, the metal sheets are fed to the tools and then processed step by step. Depending on the type of tool and the feed system, the sheets are moved only in the Y direction or in the X and Y directions in an orthogonal coordinate system. Again, sheets as large as possible are processed for reasons of productivity. When larger sheets are used, there are fewer sheets that must be lacquered, cut, fed, etc. In any case, however, the metal sheets must be brought into each punch position with a high precision.
The metal sheets are fed to the system in the form of a stack. Several thousand metal sheets may be stacked in a stack. Weight usually limits the height of the stack. The stacks are placed on a chain conveyor or a roller conveyor and then conveyed either in the production direction of the metal sheet processing in the press or in a direction 90° to the production direction into the stacking and destacking position. An electrically or hydraulically driven lift system lifts the stack up to a vertical position, in which the metal sheets are gripped individually and fed in the direction of the sheet press.
Because of the great weight of the stack (usually several tons), the stacks can be guided into the destacking position only with a relatively great inaccuracy. Consequently, the destacking position may vary as much as a few centimeters from one stack to the next. Furthermore, the stacks often have stacking errors. The metal sheets may be displaced (fanned out) either in the stacking operation in the coil cutting system or in the lacquering system or during conveyance to the processing system. Modern destacking systems today must be capable of compensating for such errors. However, more accurate centering of the stack would require complex and expensive mechanisms. Furthermore, the process of destacking each individual sheet by using suction grippers is also relatively inaccurate. When the metal sheets are gripped and conveyed in the direction of the sheet press, the sheets may as well be oiled. Rollers, which can cause additional inaccuracies in the lateral position of the sheets, are generally used for this oiling process.
Before a metal sheet fed to the punch press can be processed in the pressing tool, it must thus be brought into a defined precise position by an aligning arrangement. This is true in the feed direction to the punch press as well as in the direction perpendicular thereto. The precision must be in the range of ±0.2 mm. One reason for this is that the various punch positions in the metal sheet should be as close together as possible, so that maximum utilization of material of the metal sheet is achieved. Additional reasons may include the fact that the sheets may be lithographed (printed) and the punching must correspond precisely to the printing or that the residual web width at the edge of the sheet should be kept very small. This is an important criterion in processing metal sheets on a sheet press with a multiple tool to achieve a high productivity.
FIG. 1 shows mutually perpendicular X and Y axes, used to describe how the respective metal sheets are fed to the punch press and how the metal sheets are centered and to explain the prior art as well as explain the present invention. The destacker 2 and the press 20 are also shown schematically in FIG. 1. The metal sheets are centered in X and Y directions according to the prior art, for example, as shown schematically in FIGS. 2 through 6. A metal sheet removed from the sheet metal stack is aligned on a centering table 15, which has conveyor belts 3, 4 and 5. FIG. 3 shows that a metal sheet 1 removed from the sheet metal stack (not shown) by the destacker 2 has been deposited on the centering table. The metal sheet is conveyed by means of the conveyor belts 3, 4 and 5 on the centering table 15 in a positive Y direction (Y+ direction), which is defined as the direction toward the press 20. FIG. 4 shows the sheet 1 in a position in which it has been advanced in the Y+ direction far enough that the sheet 1 rests only on the conveyor belt 5. This is a return position from which the sheet 1 is moved by the conveyor belt 5 in the negative Y direction (Y−) for centering and/or alignment, i.e., it is moved a short distance again towards the destacker 2. FIG. 5 shows how the metal sheet 1 is in contact with two stops 6 and 7 after being conveyed with the conveyor belt 5 in the Y− direction, the stops having been raised above the level of the table for this purpose. The sheet is thus centered, i.e., aligned in the Y axis. Then a stop 8 is positioned close to the sheet in the X+ direction and finally a slide and/or another stop 9 moves the sheet in the negative X direction (X−) toward the stop 8, so that it reaches the centered and/or aligned position of the metal sheet 1 according to FIG. 6. The alignment in X and Y directions can be combined with another embodiment according to the prior art, for example, by the fact that a spring-mounted vacuum gripper system moves the sheets in the direction of a stop X+ and Y−.
DE 43 45 184 A1 discloses a centering mechanism by which a centering station, referred to here as an alignment station, for metal sheets is designed with two lateral stops and a rear stop for the metal sheets.
As soon as a metal sheet has reached its definitive orientation and position and/or is aligned correctly, it is taken over by a handling system and conveyed for processing in the tool 20. FIG. 6 therefore shows a gripper 11, which grips the sheet 1 in its previously centered/aligned position at its rear end and feeds it in the direction Y+ to the press 20. The gripper, which shifts the sheet metal in the Y+ direction, and optionally moves it in the X− and X+ directions to achieve the desired punch pattern, may be the same gripper which has gripped the centered sheet 1 in the position shown in FIG. 6. It is known that a second gripper may be present to receive the sheet 1 from the first gripper upstream from the press 20.
The alignment and/or centering mechanism according to the prior art, as explained here, is complex and requires high maintenance. Furthermore, this mechanism needs a certain amount of time in order to align the sheets. During the centering and/or alignment operations, the handling system and/or the gripper cannot yet receive the metal sheet—first, because the alignment is not yet completed, and second, because the alignment mechanism could cause a collision with the gripper.